Airborne observations of ice-nucleating particles in the vicinity of developing deep convective clouds during the North American monsoon

Deep convective clouds play crucial roles in atmospheric processes, generating lightning, severe weather, and significant rainfall, while their extensive anvils reflect solar radiation. However, models face limitations due to a lack of understanding of microphysical processes in these clouds. Ice-nucleating particles (INP), essential for initiating primary ice production, have only rarely been measured in air directly relevant for convective clouds. This makes separating the roles of primary and secondary ice difficult to resolve. Here we report the abundance and likely composition of INP during the Deep Convective Microphysics Experiment (DCMEX) campaign in New Mexico, USA, using measurements made from the FAAM BAe 146 aircraft during flights over and around the Magdalena Mountains. Orographic convective clouds frequently form directly above these mountains during the monsoon season (July-August), making the locality uniquely suited for sampling the aerosol, including INP, that become entrained into the clouds. INP were collected on filters during sampling circuits around the mountain range at varying altitudes and then analysed offline for immersion mode ice-nucleating activity using droplet freezing assays. Repeated measurements over a period of weeks enabled us to observe changes in the INP population with changes in airmass origin and also the vertical INP profile.

Overall INP concentrations observed were high (0.1 – 1 L^-1 at -10 °C) but consistent with previous observations of INP in dominantly continentally influenced air, with some INP active up to -5 °C frequently observed. Vertically resolved sampling revealed a deep and consistently present coarse aerosol layer extending from 0.5km up to 3km above ground, within which we found that the INP were evenly distributed.

Aerosol number and size-resolved compositional properties, derived using data from underwing optical probes and filter analysis with scanning electron microscopy with energy dispersive spectroscopy (SEM-EDX) respectively, were then related to the INP activity of our samples to infer composition and origin. When comparing our samples to laboratory parameterisations of aerosol classes’ ice-nucleating activity, mineral dust could account for the INP activity seen at low temperatures but were too active at higher temperatures, instead more consistent with fertile soil dust.

Throughout the campaign, there was a change in air mass origin from the northwest to the southeast and back again, however this shift did not significantly affect the INP population. When comparing our INP spectra to the parametrization of primary ice crystal number concentration by Cooper (1986), it was noted that overall, it predicts the range of our INP observations well but does not capture the observed curved shape of INP spectra at higher temperatures.

This study underscores the persistent presence of INP in growing deep convective clouds, providing insights to refine microphysics in cloud models. Comparisons with actual cloud microphysical observations would confirm primary and secondary ice production processes.